What is SU Carburetor? and How It Works?

What is SU Carburetor?

SU carburetor is an example of a constant vacuum type of carburetor. It consists of a single jet in which a tapered needle operates. The area of the throat is varied by means of a piston that slides up and down. The tapered needle is connected to the accelerator.

Working of SU Carburetor

When the accelerator is operated, the piston moves up and down in the throat controlling the supply of air, and the needle moves up and down to the jet controlling the supply of fuel.

When the piston moves up, the throat area increases, for more air passes through it. The needle also moves up increasing the annular area in the jet to pass more fuel. 

When the piston moves down, the throat area decreases, less air passes through it. The needle also moves down decreasing the annular area in the jet to pass less fuel.

The piston tapered needle is so designed that they maintain the correct air-fuel mixture at the different operating condition of the engine. By changing the tapered needle and adopting a suitable one, the same carburetor can be used on different H.P. engines to some extent.

• The upper side of the piston is connected to the throttle passage through a slot cut in the piston.
• The lower side the exposed to the atmospheric pressure.

Thus, the piston at any instant depends upon the balance of its own weight (downward) against the vacuum force (upward). As the load of the piston is constant the vacuum also remains constant.

The jet can be moved up and down about to the tapered needle, by an adjusting screw fitted at the bottom of the screw. This is done for adjusting the mixture strength. There is to separate idling or slow speed system and on accelerating pump

Types of SU Carburetor

SU carburetors were provided in several “throat sizes” in both inch and millimeter measurements. These carburetors are identified by letter prefix which indicates the float type. “H”, “HD”, “HS”, “HIF”, “HV”, “OM” and “KIF”.

“H” types have float bowls with an arm cast into their base. It is mounted to the bottom of the carburetor with a hollow bolt. The fuel passes in the carburetor body through the arm.

“HD” it has the float bowl with its arm fastening directly below, and concentric with the jet. The arm has a flange that is tightened with four screws at the bottom of the carburetor. A sealed rubber diaphragm is integral to the jet.

“HS” The main body is covered with rubber, fuel is supplied by an external pipe to the jet.

“HIF” It has a horizontal Integral Float.

“HV”, “OM” and “KIF” types are less commonly used.

Advantages and Uses

The main advantage of the SU carburetor is the rapid response during acceleration. Hence it is fitted with racing cars and in most scooters and motorcycles.

SU Carburetors were widely used in automobile vehicles from much of the twentieth century. These are also used in aircraft engines including the early versions of the Rolls Royce.

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